Power semiconductor devices are widely used to carry large currents, support high voltages and/or operate at high frequencies such as radio frequencies. A wide variety of power semiconductor devices are known in the art including, for example, High Electron Mobility Transistors (HEMT) and Metal Semiconductor Field Effect Transistors (MESFETs). Modern power semiconductor devices are generally fabricated from wide bandgap semiconductor materials. For example, HEMTs may be fabricated from silicon or, more recently, from gallium nitride (GaN) material systems on a silicon carbide (SiC) substrate. The power device may be marketed as a discrete power device or may be integrated in a Monolithic Microwave Integrated Circuit (MMIC). An overview of GaN-on-SiC HEMTs and MMICs may be found in the invited paper entitled “A Review of GaN on SiC High Electron-Mobility Power Transistors and MMICs” by Pengelly et al., IEEE Transactions on Microwave Theory and Techniques, Vol. 60, No. 6, June 2012, pp. 1764-1783.
Field effect transistors may be classified into depletion mode and enhancement mode types, corresponding to whether the transistor is in an ON-state or an OFF-state at zero gate voltage. In enhancement mode devices, the devices are OFF at zero gate voltage, whereas in depletion mode devices, the device is ON at zero gate voltage. HEMTs and MESFETs are typically depletion mode devices, in that they are conductive at zero gate bias due to the polarization-induced charge at a barrier/channel interface. Examples of such depletion mode devices include gallium arsenide (GaAs) MESFETs, GaAs pHEMTs and GaN HEMTs. Depletion mode GaN HEMTs that are marketed by Cree, Inc. are described in the following data sheets: “CMPA2735075D-75 W, 2.7-3.5 GHz, GaN MMIC, Power Amplifier” (Rev 1.1-April 2012); “CMPA0060025D-25 W, 20 MHz-6.0 GHz, GaN MMIC, Power Amplifier” (Rev 1.2-December 2014); “CMPA1D1E025F-25 W, 13.75-14.5 GHz, 40 V, Ku-Band GaN MMIC, Power Amplifier” (Rev 0.0-March 2015); “CMPA801B025D-25 W, 8.0-11.0 GHz, GaN MMIC, Power Amplifier” (Rev 1.0-June 2014); “CMPA2560025D-25 W, 2.5-6.0 GHz, GaN MMIC, Power Amplifier” (Rev 1.3-September 2012); “CMPA1D1E030D-30 W, 13.75-14.5 GHz, 40 V, GaN MMIC, Power Amplifier” (Rev 0.0-April 2015); and “CMPA5585025F-25 W, 5.5-8.5 GHz, GaN MMIC, Power Amplifier” (Rev 3.2-March 2015). Other depletion mode power devices may also be provided, such as depletion mode power Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) or Junction Field Effect Transistors (JFETs).
Sheet resistance of a GaN HEMT may be dependent on drain current. This dependence may be due to the quasi saturation of electron velocity. As such, the source sheet resistance may be dependent on drain current. For example, brief reference is now made to FIG. 1, which is a plot that illustrates GaN source sheet resistance as a function of current. In some embodiments, testing dynamic source resistance may vary from 0.4 ohms at low drain currents to 2.2 ohms at higher drain currents, such as, for example, 1 A/mm As such the dynamic source resistance may provide a significant source of non-linearity. The non-linearity may provide that signals at more than one frequency may mix together to generate cumulative signals at unwanted frequencies.
One conventional approach to addressing the effects of the dynamic source resistance may be to reduce the magnitude by reducing the gate-source spacing, using, for example, self-aligned structures. However, such approaches may present other problems including, for example, lower breakdown voltages.